It is known that biological materials, including human and animal tissue and blood, have dielectric properties that vary somewhat with the type of tissue. These dielectric properties include permittivity, and conductivity.
Tissue-specific dielectric properties have historically often been measured with a probe formed from a straight section of coaxial transmission line having an open, unterminated, end that is applied to the tissue. A coaxial probe of this type having diameter of 0.085 inch is, for example, described for use between 10 MHz and 10 GHz in T. Whit Athey, et al. IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-28, NO. 4, APRIL 1980, Measurement of Radio Frequency Permittivity of Biological Tissues with an Open-Ended Coaxial Line: Part I. Using such probes, dielectric properties of tissues were found to differ between kidney, muscle, brain, fat, and other tissue types.
Using coaxial probes of various designs, dielectric properties of tissue in the range 10 kHz to 1 MHz have been shown to differ between cancerous and non-cancerous tissues within a human breast, as reported in Ryan J Halter, et al. Physiol. Meas. 30 (2009) S121-S136 The correlation of in vivo and ex vivo tissue dielectric properties to validate electromagnetic breast imaging: initial clinical experience (Halter). The intraoperative coaxial probe used in-vivo by Halter was limited to the 10 to 100 kHz range, however measurements on removed breast tissue indicated that differences in dielectric properties of at least some tumor types and breast stroma also exist at higher frequencies into at least the low MHz range.
There are potential advantages of measuring electrical parameters of tissue in the microwave frequency range, in particular between 100 MHz and 10 GHz. At lower frequencies, differences between permittivity and conductivity may be related to intracellular and extracellular water. At these higher frequencies above 100 MHz, it is possible to quantify total tissue water and to discriminate between free and chemically-bound water in the tissue.